Accept constant string concatenation and constant string length as constant

runtime/vm/ast.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_AST_H_
 #define VM_AST_H_
 
 #include "platform/assert.h"
 #include "vm/allocation.h"
 #include "vm/growable_array.h"
@@ skipping 111 matching lines @@
   // parsing as the assignment context was not known yet at that time.
   virtual AstNode* MakeAssignmentNode(AstNode* rhs) {
     return NULL;  // By default all nodes are not assignable.
   }
 
   // Return NULL if 'unary_op_kind' can't be applied.
   virtual AstNode* ApplyUnaryOp(Token::Kind unary_op_kind) {
     return NULL;
   }
 
+  // Returns true if this node can be a compile-time constant, assuming
+  // that all nodes it depends on are also compile-time constants of
+  // the proper types and values.
+  // See the concept of "potentially constant expression" in the language spec.
+  // The purpose of IsPotentiallyConst is to detect cases where the node is
+  // known NOT to be a constant expression, in which case false is returned and
+  // a compile-time error is reported by the compiler. Otherwise, an error may
+  // still be reported at run-time depending on actual values.
   virtual bool IsPotentiallyConst() const { return false; }
 
   // Analyzes an expression to determine whether it is a compile time
   // constant or not. Returns NULL if the expression is not a compile time
   // constant. Otherwise, the return value is an approximation of the
   // actual value of the const expression. The type of the returned value
   // corresponds to the type of the const expression and is either
   // Number, Integer, String, Bool, or anything else (not a subtype of
   // the former).
   virtual const Instance* EvalConstExpr() const { return NULL; }
@@ skipping 1274 matching lines @@
 
   AstNode* receiver() const { return receiver_; }
   const String& field_name() const { return field_name_; }
 
   virtual void VisitChildren(AstNodeVisitor* visitor) const {
     receiver()->Visit(visitor);
   }
 
   virtual AstNode* MakeAssignmentNode(AstNode* rhs);
 
+  virtual bool IsPotentiallyConst() const;
+  virtual const Instance* EvalConstExpr() const;
+
   DECLARE_COMMON_NODE_FUNCTIONS(InstanceGetterNode);
 
  private:
   AstNode* receiver_;
   const String& field_name_;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(InstanceGetterNode);
 };
 
 
@@ skipping 468 matching lines @@
   const intptr_t try_index_;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(InlinedFinallyNode);
 };
 
 }  // namespace dart
 
 #undef DECLARE_COMMON_NODE_FUNCTIONS
 
 #endif  // VM_AST_H_